Yes, Muay Thai can be physically demanding
if you try to go balls to the walls all out but the better MT gyms will let you work at your own pace and not expect you to become Ramon Dekkers overnight.
Like I said earlier. They will teach you proper technique then in time you will develop speed and power.
At my gym we have 3 50+ students one is a 56 year old lady (5 foot 3) who does boxing as well as MT classes 3x a week. She started 3 years ago with no martial arts training. She struggled in the beginning (like all of us did myself included) but now she makes it through each class no problem. I hold the pads for her and I'll tell you. She hits pretty hard for her age.
Sparring is optional and actually you would have to be at a certain level before you can get in the ring and spar so it's unlikely that you will get hurt besides maybe extremely sore muscles starting out.
Classes are usually made up of endless various punches to the focus pads (overhand right, jab's, left hook, uppercut, body shots etc), different kick's to the pads, kneeing the pads, learning 2,3,4,5,6 strike combo's, footwork, weaving, slipping and rolling off the jab etc.etc. All the basic stuff. It's quite a workout and you will sweat buckets but you will get in shape, lose weight, tone up drastically and know how to defend yourself to some degree.
You can start MT at age 50 just take it easy for a while and work on learning technique. If you're just starting out and cant finish the class then go as long as you can and sit out the rest. It's no problem. We were all there before. You do what you can.